1. Field of the Invention
The present invention relates to a process for separating mixtures of hydrocarbon isomers in gas phase on molecular sieves.
2. Discussion of the Background
The separation of the constituents of a mixture of hydrocarbon isomers, with very similar boiling points, is traditionally based on suitable technical combinations of superfractionation and crystallization processes, with high costs and poor yields.
Alternatively, it is possible to obtain the separation of one or more isomers more economically and efficiently by means of adsorption processes on molecular sieves carried out in liquid phase (U.S. Pat. No. 3,917,734, U.S. Pat. No. 3,998,901); in these systems, the fluid and adsorbing solid are put in contact with each other in countercurrent. The movement of the solid can be either effective or simulated, the latter being effected by continuously varying the position of the feeds and sampling over a period of time.
For some hydrocarbon groups (paraffins-olefins, C4-C5 cuts), there are also processes on molecular sieves, carried out at such temperature and pressure values as to ensure a feeding mixture in vapor phase (U.S. Pat. No. 5,563,299).
These processes consist of two sections:                passage of the mixture on a fixed bed of molecular sieves where the isomers with a greater selectivity are withheld and the remaining ones are eluted (refined product);        recovery of the withheld isomers by desorption with a desorbing agent in vapor phase (extracted product). The continuous unit which effects this process therefore comprises the installation of at least two fixed molecular sieve beds (or multiples of two, i.e. 4, 6, 8, etc.) operating alternatively in adsorption and desorption.        
This process is completed by distillation operations for the recovery of the refined product and extracted product from the desorbing agent; in fact it should be pointed out that the bed in adsorption phase comes from the previous desorption phase, at the end of which it is completely saturated with desorbing agent.
We have now found a process using the vapor phase with any mixture of hydrocarbon isomers which allows a greater recovery per cycle of the desired product in the refined product.